Add NaOH. mols added = M x L. NaOH reacts with acetic acid to produce more acetate at the expense of the acetic acid. Therefore, calulate moles NaOH added, subtract from acetic acid and add to acetate. Then M = moles/L, substitute base and acid into Henderson-Hasselbalch equation and solve for pH. I get something like 4.9 but that's an estimate and you need to confirm it with more accurate calculations.